1. Field of the Invention
This invention is directed to telephone system accessories, in general, and to intercom systems which are modularly connectable with existing telephone lines, in particular.
2. Prior Art
There are many types of intercom systems on known in the prior art. Some intercom systems are stand alone or separate units with transmitter/receiver units at different locations. The systems often require the use of separate interconnecting lines which must be provided and installed between the separate units. The problems with this type of installation in existing structures is inherently obvious.
There are other intercom systems which are purported to be operable when the respective units are plugged into existing wiring in an existing structure. However, these units have the shortcoming that they must operate on the existing line voltage, will frequently be inoperative when the line voltages are disconnected and require separate and additional units at the respective locations. Moreover, these units frequently are subject to interference, such as 60 Hz hum, on existing household circuitry. In addition, many existing telephone systems in the home or small office include a single line telephone with multiple extensions. It is possible with these extensions to effect an intercom system by merely lifting and speaking through to extension phones or handsets. However, this approach has the distinct disadvantage that the lifting of one or more of these handsets triggers the dial tone which is provided by the central office of the telephone system. In addition, there is no reliable signalling apparatus to assure that the appropriate extension is answered by the appropriate person with whom communication is desired. On the other hand, any attempt to provide reliable signalling will usually grossly encumber the existing system and entail the problems discussed above relative to separate intercom systems.
In other intercom systems which are supplied with existing telephones there is usually a requirement of a separate wiring system for the signalling, such as a buzzer or the like, and the requirement of additional push buttons on the instrument. The provision of separate hold buttons or push buttons on telephone instruments are also considered separately billable items by most telephone companies so that the expense for such items is not only large at the initial installation, but the rental cost is continuous.
Moreover, these systems are usually not exchangeable or easily modified in the case of telephone intercom systems which are included in the existing phone instrument.